Thin-diameter coaxial cable and method of producing the same

ABSTRACT

A small-diameter coaxial cable comprises a central conductor, an insulated covering layer arranged on the outer periphery of the central conductor and having air gaps continuous along the longitudinal direction and an outer conductor layer arranged on the outer periphery of the insulated covering layer. The insulated covering layer includes an inner annular portion covering the outer periphery of the central conductor, a plurality of coupling portions extending outward from the inner annular portion and an outer annular portion connecting the outer peripheral edges of the coupling portions to each other. Air gaps defined along the peripheral direction by the coupling portions are formed on the inner side of the insulated covering layer.

TECHNICAL FIELD

The present invention relates to a small-diameter coaxial cable beingsuperior in high-frequency characteristic and electrical characteristicsand a method of fabricating the same.

BACKGROUND ART

With the recent trend toward an increased information volume and anincreased transmission speed, the coaxial cable has begun to be used forthe antenna wire of the portable information terminals and the wiresconnecting the LCD and the CPU. On the other hand, the reduced size andthickness of the information terminals and notebook-sized personalcomputers requires a smaller diameter of the coaxial cable. Generally,to acquire the coaxial cable having superior electrical characteristics,it is crucial to decrease the dielectric constant of an insulatedcovering layer formed on the outer periphery of the central conductor asmuch as possible. For this purpose, the insulated covering layer isoften formed of fluoro resin or polyolefin resin which is low indielectric constant. Also, the foaming process is often employed toreduce the apparent dielectric constant.

In the case of the foaming extrusion technique, however, it is difficultto secure the extrusion stability. Especially, in the extrusion of asmall-diameter component, the outer diameter of the insulated coveringlayer undergoes a delicate change, thereby causing the variation in thehigh-frequency characteristic and the electrical characteristics.

The coaxial cable can be effectively reduced in diameter, on the otherhand, by using a metal plating layer instead of a braided metal wire asan outer conductor formed on the outer periphery of the insulatedcovering layer.

In the case where the insulated covering layer is foamed to reduce theapparent dielectric constant, however, the problem is posed that theplating solution intrudes into the bubbles of the foamed portion for anincreased dielectric constant or corrodes the outer conductor therebyadversely affecting the electrical characteristics of the coaxial cable.

This invention has been developed in view of these problems of the priorart and the object thereof is to provide a small-diameter coaxial cablehaving the frequency characteristic and the electrical characteristicswhich are both superior and stable.

DISCLOSURE OF THE INVENTION

In order to achieve the object described above, according to thisinvention, there is provided a small-diameter coaxial cable comprising acentral conductor, an insulated covering layer arranged on the outerperiphery of the central conductor and having air gaps continuous alongthe longitudinal direction and an outer conductor layer arranged on theouter periphery of the insulated covering layer, characterized in thatthe insulated covering layer includes an inner annular portion coveringthe outer periphery of the central conductor, a plurality of couplingportions extending outward from the inner annular portion and an outerannular portion connecting the outer peripheral edges of the couplingportions to each other, and the coupling portions define the peripheraldirection of the air gaps.

In the small-diameter coaxial cable having this configuration, theprovision of the air gaps defined by the coupling portions in theinsulated covering layer reduces the equivalent dielectric constant andimproves the high-frequency characteristic and the electricalcharacteristics.

This improvement can be attained without the expansion molding andtherefore a high accuracy of outer diameter is obtained. At the sametime, the elimination of the need of sizing makes possible a high-speedmolding process. In the case where a plating layer (outer conductor) isformed on the outer annular portion, there is no likelihood that theplating solution intrudes in the bubbles thereby corroding the outerconductor.

In the small-diameter coaxial cable having this configuration, the innerannular portion and the coupling portions combined with the outerannular portion, the inner annular portion combined with the couplingportions and the outer annular portion, or the outer annular portion canbe formed into double layers of different types of resin.

With this configuration, the insulated covering layer is preferablyformed of fluoro resin having a small dielectric constant. The fluororesin, however, has a low adhesion with the plating film and poses aproblem in the case where the outer conductor layer is formed byplating. Nevertheless, the plating performance can be improved byemploying a resin having a high adhesion with the plating film as amaterial (thermoplastic resin) of the outer annular portion. In such acase, the outer annular portion is preferably constructed in doublelayers including an outer layer and an inner layer of differentmaterials.

Preferably, in the small-diameter cable having the configurationdescribed above, the outer annular portion is formed of a resin capableof being plated with a metal, and the outer conductor layer can beformed by metal plating.

With this configuration, each strand of an ordinary stranded shield wireor a braided shield wire cannot be reduced in diameter to less than thelimit of about 25 μm, for example. Also, when the wire is bent, thestrands may be loosed open to form gaps, causing a signal leakage. Inthe case where the outer conductor layer is formed by metal plating,however, the thickness of the conductor layer can be reduced andtherefore the diameter of the coaxial cable can be further reduced.Also, no gap is formed when the wire is bent.

Also, according to this invention, there is provided a small-diametercoaxial cable comprising a central conductor, an insulated coveringlayer arranged on the outer periphery of the central conductor andhaving air gaps continuous along the longitudinal direction and an outerconductor layer arranged on the outer periphery of the insulatedcovering layer, characterized in that the insulated covering layerincludes an annular portion covering the outer periphery of the centralconductor and one or more columnar (rib) portions extending outward fromthe annular portion, the outer conductor layer is arranged to be incontact with the outer periphery of the columnar portions, and one ormore air gaps continuous along the longitudinal direction are formed onthe inner side of the outer conductor layer.

With this configuration, one or more air gaps continuous along thelongitudinal direction can be formed on the inner side of the outerconductor layer, and the equivalent dielectric constant between thecentral conductor and the outer conductor layer (insulated coveringlayer) can be reduced.

The outer conductor layer of the small-diameter coaxial cable having theconfiguration described above can be formed of a hollow compressedstranded wire.

With this configuration, the hollow compressed stranded wire (hollowstranded wire) has a self-supporting structure, and therefore cancontain a linear object of an arbitrary shape having an outer diametersmaller than the inner diameter thereof. Also, by providing an insulatedcovering layer having a rib on the central conductor, the centralconductor can be arranged at the center of the hollow stranded wire.

The hollow stranded wire has the strands in close contact with eachother, and therefore forms no gap between the strands when bent. Also,since the strands are not bonded to each other, the flexibility isbasically high.

The outer conductor layer of the small-diameter coaxial cable having theconfiguration described above can be formed in such a manner that ametal tape or a metal foil having a superior electrical conductivitysuch as copper or a metal laminate film produced by laminating the metaltape or the metal foil with a plastic film is wound on the outerperiphery of the columnar portion.

With this configuration, the outer conductor layer is formed in such amanner that a metal tape or a metal foil having a superior electricalconductivity such as copper or a metal laminate film produced bylaminating the metal tape or the metal foil with a plastic film is woundon the outer periphery of the columnar portion, and therefore, thesmall-diameter coaxial cable can be formed with relative ease using asimple means.

The small-diameter coaxial cable having the configuration describedabove can be formed in such a manner that a semi-finished product(insulating core) including the outer conductor layer of a metal pipehaving a superior electrical conductivity such as copper and a coveringlayer having the columnar portions formed on the outer periphery of thecentral conductor is inserted into the metal pipe while at the same timedrawing the metal pipe through a die.

With this configuration, a semi-finished product (insulating core)including the outer conductor layer of a metal pipe having a superiorelectrical conductivity such as copper and a covering layer having thecolumnar portions formed on the outer periphery of the central conductoris inserted into the metal pipe while at the same time drawing the metalpipe through a die, and therefore the small-diameter coaxial cable canbe formed with relative ease.

In the small-diameter coaxial cable having the configuration describedabove, a plurality of the coupling portions and the columnar portionsextend radially at equal angular intervals in the cross section and canbe extended along the longitudinal axial direction of the small-diametercoaxial cable at the same angular intervals.

Also, the coupling portions and the columnar portions of thesmall-diameter coaxial cable having the configuration described abovecan be formed spirally along the longitudinal direction.

With these configurations, a plurality of the air gaps can be uniformlyarranged along the peripheral direction around the central conductor.The air gaps, thus arranged uniformly, have a superior forming stabilityand a superior geometric accuracy. The columnar portions may be formedspirally by rotating the covering die.

In the small-diameter coaxial cable having the configuration describedabove, the annular portion, the coupling portions and the columnarportions can be formed by extruding fluoro resin such as FEP, PFA orPTFE or the synthetic resin such as APO (amorphous polyolefin) resin orPEN (polyethylene naphthalate).

With this configuration, the insulated cover is formed of fluoro resinselected from PFA (tetrafluoroethylene-Perfluoroalkyl vinyl ethercopolymer), FEP (tetrafluoroethylene-hexafluoropropyrene copolymer) orPTFE (polytetrafluoroethylene), amorphous polyolefin resin orpolyethylene naphthalate, and therefore the relative dielectric constantis low (3 or less) and the heat resistance is high.

The insulated covering layer of the small-diameter coaxial cable havingthe configuration described above can occupy at least 10% in area of theair gaps in the cross section.

With this configuration, the air gaps occupy the area of at least 10% ofthe insulated covering layer in cross section. By increasing thehollowness of the air gaps, the equivalent dielectric constant can bereduced. Preferably, therefore, the hollowness is increased to at least50%, up to an upper limit of 90%, from the viewpoint of strength(passability through the processes) of the insulated covering layer.

In the small-diameter coaxial cable having the configuration describedabove, a protective covering layer can be formed on the outer peripheryof the outer conductor layer so that the outermost diameter of thesmall-diameter coaxial cable can be set to not more than 1 mm.

According to this invention, there is provided a method of fabricating asmall-diameter coaxial cable, characterized in that a covering diehaving a central hole for insertion of the central conductortherethrough and including a resin discharge portion having, a circularannular portion formed on the outer periphery of the central hole and aplurality of radial slits extending radially outward from the outerperiphery of the circular annular portion is used in such a manner thatthe central conductor is inserted through the central hole while at thesame time molding by extruding the melted thermoplastic resin, with adraft, from the resin discharge portion thereby to obtain anintermediate molded component including an inner annular portioncovering the outer periphery of the central conductor and a plurality ofcoupling portions extending outward from the inner annular portion andsimilar in shape to the die, after which the intermediate moldedcomponent is introduced to the head of a melt extruder, and the outerannular portion is covered by extrusion on the coupling portions by anannular covering die thereby to form the insulated covering layer havingthe air gaps, after which the outer conductor layer and the protectivecovering layer are sequentially formed on the outer periphery of theinsulated covering layer.

In the method of fabricating the small-diameter coaxial cable having theconfiguration described above, the insulated covering layer is formed intwo stages. Since the layer is covered with a draft, the resin dischargeportion of the die can be larger than the (intermediate) moldedcomponent. In this case, the draft makes it possible to position thecentral conductor easily at the center for an improved geometricaccuracy while at the same time increasing the molding speed by reducingthe discharge pressure.

Also, according to this invention, there is provided a method offabricating a small-diameter coaxial cable, characterized in that thecentral conductor is covered by extrusion, with a draft, with thethermoplastic resin melted in annular fashion by an annular covering diethereby to obtain an intermediate molded component having an innerannular portion covering the outer periphery of the central conductor,after which using a die including a central hole and a resin dischargeportion having an annular portion and a plurality of radial holesextending radially from the inner periphery of the annular portion, theintermediate molded component is inserted through the central hole whileextruding the melted thermoplastic resin from the resin dischargeportion with a draft thereby to form an outer annular portion and aplurality of coupling portions extending to the center, thereby formingthe insulated covering layer having the air gaps, after which the outerconductor layer and the protective covering layer are sequentiallyformed and covered on the outer periphery of the insulated coveringlayer.

With this configuration, unlike the invention described in claim 13, thecoupling portions and the outer annular portion are integrated andmolded with a draft. In the process, the draft makes it possible toincrease the size of the resin discharge portion of the die as comparedwith the shape of the (intermediate) molded component.

In this case, the draft makes it possible to position the centralconductor at the center easily for an improved geometric accuracy. Atthe same time, the lower discharge pressure can increase the moldingrate.

In the method of fabricating the small-diameter coaxial cable having theconfiguration described above, as an alternative to the process ofproducing the intermediate molded component, the dispersion with thethermoplastic resin particles dispersed in a dispersion medium (liquid)is coated or impregnated around the central conductor, after which thedispersion medium is evaporated thereby to form an annular covering onthe central conductor or an annular covering is formed by powder coatingthereby to form the inner annular portion and obtain an intermediatemolded component having the inner annular portion covering the outerperiphery of the central conductor.

With this configuration, the thickness of the annular covering aroundthe central conductor can be reduced as compared with the thickness (toa limit of about 30 μm) obtained by the extrusion covering.

Further, according to this invention, there is provided a method offabricating a small-diameter coaxial cable comprising a centralconductor, an insulated covering layer arranged on the outer peripheryof the central conductor and having air gaps continuous along thelongitudinal direction, an outer conductor layer arranged on the outerperiphery of the insulated covering layer and a protective coveringlayer arranged on the outer periphery of the outer conductor layer,characterized in that using a die having a central hole for insertingthe central conductor therethrough and a plurality of T-shaped splitholes arranged adjacently to each other on the outer periphery of thecentral hole, the central conductor is inserted through the central holewhile at the same time extruding the melted resin from the central holeand the T-shaped split holes thereby to form the insulated coveringlayer having the air gaps continuous along the longitudinal direction onthe outer periphery of the central conductor, after which the outerconductor layer and the protective covering layer are sequentiallyformed and covered on the outer periphery of the insulated coveringlayer.

With this configuration, using a die having an insertion hole for thecentral conductor and a plurality of T-shaped split holes arrangedadjacently to each other on the outer periphery of the central hole, thecentral conductor is inserted into the central hole while at the sametime extruding the melted resin from the central hole and the splitholes thereby to form the insulated covering layer having the air gapscontinuous along the longitudinal direction on the outer periphery ofthe central conductor in one stage.

In the method of fabricating the small-diameter coaxial cable having theconfiguration described above, the outer conductor layer can be formedby metal plating.

The metal plating is conducted in such a manner that the surface of theinsulated covering is roughened and subjected to the hydrophilicprocess, after which the electroless plating and the electrolyticplating are conducted to form the outer conductor layer.

Also, according to this invention, there is provide a method offabricating a small-diameter coaxial cable, characterized in that acovering die including a central hole for inserting the centralconductor therethrough and a resin discharge portion having an annularportion and a plurality of radial slits extending radially outward fromthe outer periphery of the annular portion is used in such a manner thatthe central conductor is inserted through the central hole while at thesame time molding by extrusion, with a draft, the melted thermoplasticresin from the resin discharge portion thereby to obtain an intermediatemolded component (insulated core) similar in shape to the die and havingan inner annular portion covering the outer periphery of the centralconductor and a plurality of coupling portions extending outward fromthe inner annular portion, which intermediate molded component issupplied continuously so that an outer conductor layer is formed bycovering a hollow compressed stranded wire or winding a metal foil, alaminate film or the like or covering by extending a copper pipe on theouter periphery of the columnar portions, after which an outer coveringlayer is formed on the outer periphery of the outer conductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a small-diameter coaxial cableaccording to a first embodiment of the invention;

FIG. 2 is a perspective view showing a small-diameter coaxial cableaccording to a second embodiment of the invention;

FIG. 3 is a sectional view showing a small-diameter coaxial cableaccording to a third embodiment of the invention;

FIG. 4 is a sectional view showing a small-diameter coaxial cableaccording to a fourth embodiment of the invention;

FIG. 5 is a sectional view showing a small-diameter coaxial cableaccording to a fifth embodiment of the invention;

FIG. 6 is a perspective view showing a small-diameter coaxial cableaccording to a sixth embodiment of the invention;

FIG. 7 is a diagram for explaining the covering die used in a firstspecific example of the method of fabricating the small-diameter coaxialcable according to this invention;

FIG. 8 is a sectional view for explaining an intermediate moldedcomponent obtained during the fabrication in the first specific exampleof the method of fabricating the small-diameter coaxial cable accordingto this invention;

FIG. 9 is a sectional view for explaining a second intermediate moldedcomponent obtained during the fabrication in the first specific exampleof the method of fabricating the small-diameter coaxial cable accordingto this invention;

FIG. 10 is a diagram for explaining the covering die used in a thirdspecific example of the method of fabricating the small-diameter coaxialcable according to the invention; and

FIG. 11 is a sectional view for explaining an intermediate moldedcomponent obtained during the fabrication in the third specific exampleof the method of fabricating the small-diameter coaxial cable accordingto this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The mode for carrying out the invention is explained in more detailbelow with reference to embodiments and specific examples.

FIG. 1 shows a small-diameter coaxial cable according to a firstembodiment of the invention. The small-diameter coaxial cable shown inthe drawing comprises a central conductor 1, an insulated covering layer2, an outer conductor layer 3 and a protective covering layer 4.

The central conductor 1 is configured of a thin wire of copper or copperalloy high in strength and electrical conductivity or a solid wire or astranded wire of the thin wire plated with a metal higher in electricalconductivity. To obtain a coaxial cable smaller in diameter, however,the solid wire is desirably used.

The insulated covering layer 2 is formed of a thermoplastic resin andincludes an inner annular portion 2 a covering the outer periphery ofthe central conductor 1, four coupling portions 2 b radially extendingoutward from the outer periphery of the inner annular portion 2 a, andan outer annular portion 2 c for coupling the outer ends of the couplingportions 2 b to each other.

According to this embodiment, the four coupling portions 2 b arearranged at equal angular intervals along the peripheral direction, sothat four air gaps 5 continuous along the longitudinal direction areequidistantly arranged along the peripheral direction around the centralconductor 1. Thus, a small space of each air gap 5 is defined by thecoupling portions 2 b.

The air gaps 5 are not limited to four but may be at least one innumber, and formed in such a manner that each outer end portion thereoffails to reach the outer peripheral edge of the insulated covering layer2, i.e. the outer edge of the outer annular portion 2 c.

The insulated covering layer 2 having a plurality of the air gaps 5continuous along the longitudinal direction is fabricated by any one ofthree methods. In the first method, using a covering die comprising acentral hole for inserting the central conductor 1 therethrough and aresin discharge portion configured of, a circular annular portion and aplurality of radial slits extending radially from the outer periphery ofthe annular portion, the central conductor 1 is inserted into thecentral hole while at the same time molding by extrusion a meltedthermoplastic resin from the resin discharge portion with a draftthereby to obtain an intermediate molded component, similar in shape tothe die, including an inner annular portion 2 a covering the outerperiphery of the central conductor 1 and a plurality of the couplingportions 2 b extending outward from the inner annular portion 2 a, afterwhich the intermediate molded component is introduced to the head of amelt extruder, so that an outer annular portion 2 c is covered byextrusion on the coupling portions 2 b using the circular annularcovering die, after which an outer conductor layer 3 and a protectivecovering layer 4 are sequentially formed on the outer periphery of theinsulated covering layer 2.

In the second method, the central conductor 1 is inserted through acircular annular covering die, and the melted thermoplastic resin iscovered by extrusion in annular fashion on the outer periphery thereofwith a draft thereby to obtain an intermediate molded component havingan inner annular portion 2 a covering the outer periphery of the centralconductor 1, after which using a die including a central hole forinserting the intermediate molded component therethrough and a resindischarge portion having a circular annular portion forming an outerannular portion and a plurality of radial holes extending radially fromthe inner periphery of the annular portion toward the center, theintermediate molded component is inserted through the central hole whileat the same time extruding the melted thermoplastic resin from the resindischarge portion with a draft thereby to form an outer annular portion2 c and a plurality of coupling portions 2 b extending toward the centerso that an insulated covering layer 2 having the air gaps 5 are formed,after which an outer conductor layer 3 and a protective covering layer 4are sequentially formed on the outer periphery of the insulated coveringlayer 2.

In the third method, as an alternative to the process of producing theintermediate molded component in the second method, a dispersion withthermoplastic resin particles dispersed in a dispersion medium (liquid)is coated or impregnated around the central conductor, after which thedispersion medium is evaporated thereby to form an annular cover on thecentral conductor or to form an annular cover by powder coating. In thisway, the inner annular portion is formed and an intermediate moldedcomponent having the thin inner annular portion covering the outerperiphery of the central conductor is obtained, after which by the sameprocess as in the second method, an outer annular portion 2 c and aplurality of coupling portions 2 b extending toward the center areformed thereby to form the insulated covering layer having the air gaps,after which an outer conductor layer 3 and a protective covering layer 4are sequentially covered on the outer periphery of the insulatedcovering layer 2.

The outer conductor layer 3 is covered on the outer periphery of theinsulated covering layer 2. In the case where this outer conductor layer3 is formed by metal plating, the insulated covering layer 2 isactivated by the plasma treatment, the flame treatment, the treatment bya strong acid such as chromic acid or sulfuric acid or the etchingprocess with sulfuric acid, phosphoric acid or chromic acid (dichromicacid) aqueous solution, followed by sensitization with the hydrochloricacid solution of tin chloride, further followed by activation with thehydrochloric acid solution of palladium chloride, and then theelectroless plating is conducted.

In this case, the metal plating layer may be a double structureincluding an electroless plating anchor metal layer and an electricallyconductive metal layer formed on the outer periphery of the electrolessplating anchor metal layer.

The insulated covering layer 4 formed on the outermost periphery, thoughnot necessarily required, is formed to cover the outer conductor layer 3according to this embodiment. This insulated covering layer 4 is formedby extrusion of, for example, FEP or polyvinyl chloride resin (PVC) orcoating of acrylic resin or polyimide resin. By the way, thesmall-diameter coaxial cable shown in FIG. 1 can be reduced to asufficiently small diameter as long as the outermost diameter is notmore than 1 mm.

FIG. 2 shows a small-diameter coaxial cable according to a secondembodiment of the invention. The small-diameter coaxial cable shown inthe drawing comprises a central conductor 12, an insulated coveringlayer 14 and an outer conductor layer 16. The central conductor 12 isconfigured of, for example, a copper wire having a circular crosssection.

The insulated covering layer 14 is electrically insulative, andaccording to this embodiment, includes an annular portion 18 coveringthe outer periphery of the central conductor 12 and columnar portions 20projected from the outer periphery of the annular portion.

The insulated covering layer 14 is such that the annular portion 18 andthe columnar portion 20 can be formed integrally by extrusion molding ofthe fluoro resin such as FEP or PFA or the synthetic resin such asamorphous polyolefin resin or PEN (polyethylene naphthalate) on theouter periphery of the central conductor.

According to this embodiment, the insulated covering layer 14 includesfour columnar portions 20 each extending outward from the center and hasa generally cross-shaped cross section. The columnar portions 20 extendradially at equal angular intervals (90° C.) in the cross section, whileat the same time linearly extending with the same intervals along thelongitudinal axial direction of the small-diameter coaxial cable 10.

The outer conductor layer 16 is formed in contact with the outerperiphery of the columnar portions 20 of the insulated covering layer,and four air gaps 22 continuous along the longitudinal direction of thesmall-diameter coaxial cable 10 are defined by the columnar portions 20on the inner side of the outer conductor layer 16.

According to this embodiment, the outer conductor layer 16 is formed bya hollow compressed stranded wire. This compressed stranded wire isformed as a hollow wire by arranging a plurality of strands 24 on thesame circumference and twisting each strand 24 in one direction while atthe same time passing it through a compression die, so that the hollowshape is maintained without being deformed. The outermost diameter ofthe small-diameter coaxial cable 10 according to this embodiment can bemaintained at not more than 1 mm.

In the small-diameter coaxial cable 10 having the configurationdescribed above, the four air gaps 22 continuous along the longitudinaldirection are formed on the inner side of the outer conductor layer 16,and therefore the dielectric constant between the central conductor andthe outer conductor can be reduced.

FIG. 3 shows a small-diameter coaxial cable according to a thirdembodiment of the invention. The component parts identical or equivalentto those in the embodiments described above are designated by the samereference numerals, respectively, and not described again, and only thefeatures of them are described below.

The embodiment shown in FIG. 3 is a modification of the secondembodiment, and a protective covering layer 26 of an electricallyinsulating characteristic is formed on the outer periphery of the outerconductor layer 16 a configured of the hollow compressed stranded wireof the second embodiment. The small-diameter coaxial cable 10 a havingthis configuration also has the functions and effects equivalent tothose of the second embodiment.

FIG. 4 shows a small-diameter coaxial cable according to a fourthembodiment of the invention. The component parts identical or equivalentto those in the embodiments described above are designated by the samereference numerals, respectively, and not described again, and only thefeatures of them are described below.

The embodiment shown in FIG. 4 comprises a central conductor 12 and aninsulated covering layer 14 of the same configuration as that of thesecond embodiment, except that the outer conductor layer 16 b has afeature.

Specifically, according to this embodiment, the outer conductor layer 16b is formed of a metal tape or a metal foil having a superior electricalconductivity such as copper or a metal laminate film with the metal tapeor the metal foil laminated with a plastic film. The member selectedfrom them is wound on the outer periphery of the columnar portions 20.

In this case, the tape, etc. is wound in such a manner that no gap isformed along the longitudinal axial direction of the cable. Thesmall-diameter coaxial cable 10 b having this configuration has also thefunctions and effects equivalent to those of the second embodiment.

FIG. 5 shows a small-diameter coaxial cable according to a fifthembodiment of the invention. The component parts identical or equivalentto those of the embodiments described above are designated by the samereference numerals, respectively, and not described again, and only thefeatures thereof are described below.

The embodiment shown in FIG. 5 comprises a central conductor 12 and aninsulated covering layer 14 of the same configuration as in the secondembodiment, except that the outer conductor layer 16 c has a feature.

Specifically, according to this embodiment, the outer conductor layer 16c is formed of a metal pipe having a superior electrical conductivitysuch as copper, and a semi-finished product formed with the insulatedcovering layer 14 having the columnar portions 20 on the outer peripheryof the central conductor 12 is inserted into a metal pipe while drawingand extending the metal pipe through a die. The small-diameter coaxialcable 10 c having this configuration also has the functions and effectsequivalent to those of the second embodiment.

By the way, in the fourth and fifth embodiments shown in FIGS. 4 and 5,the protective covering layer shown in the first embodiment can beformed on the outer periphery of the outer conductor layer 16 b, 16 c.

FIG. 6 shows a small-diameter coaxial cable according to a sixthembodiment of the invention. The component parts identical or equivalentto those of the embodiments described above are designated by the samereference numerals, respectively, and not described again, and only thefeatures of this embodiment are described below.

This embodiment has the external appearance shown in FIG. 6 as asemi-finished product formed with the insulated covering layer 14 d onthe outer periphery of the central conductor 12, and the insulatedcovering layer 14 d includes an annular portion 18 d and a columnarportions 20 d.

The annular portion 18 d, like in the second embodiment, covers theouter periphery of the central conductor 12 in annular fashion. Thecolumnar portions 20 d, on the other hand, which constitute a structureof six columns extending outward from the center in such a manner as tobe wound spirally at predetermined pitches on the outer periphery of theannular portion 18 d. The columnar portions 20 d can be formed byrotating the die in one direction while extruding the melted syntheticresin. Only one columnar portion 20 may be used depending on the spiralpitch.

According to this embodiment, once either one of the outer conductorlayers 16 a, 16 b in the embodiments described above is formed on theouter periphery of the columnar portions 20 d, the spiral air gaps 22 dare formed therein, and therefore the functions and effects equivalentto those of the above-mentioned embodiments are obtained.

More specific examples of the small-diameter coaxial cable and themethod of fabrication thereof according to the invention are explainedwith reference to comparative examples. This invention, however, is notlimited to the specific examples described below.

SPECIFIC EXAMPLE 1

The central conductor (silver-plated copper wire having the outerdiameter φ of 0.1 mm) 1 was heated, so that the surface temperaturebecame 100° C., by a heater using an electric burner, introduced to across head die and inserted through a covering die (nozzle) 30 havingthe shape shown in FIG. 7.

The covering die 30 shown in FIG. 7 includes a central hole 30 a forinserting the central conductor 1 therethrough, and four radial splitholes (resin discharge holes) 30 b formed on the outer peripheral edgeof the central hole 30 a and extending radially outward.

The inner diameter of the central hole 30 a is larger than the outerdiameter of the central conductor 1, and once the central conductor 1was inserted through the central hole 30 a, predetermined gaps (resindischarge portions) were formed between the outer periphery of theconductor 1 and the central hole 30 a and the resin is discharged intothese gaps.

Also, the four slit holes 30 b had substantially the same shape as thecoupling portions 2 b and were equidistantly arranged along theperipheral direction around the central hole 30 a.

Using the covering die 30 shaped like this, the central conductor 1 wasinserted through the central hole 30 a, while at the same time beingtaken off at the rate of 30 m/min. At the same time, the cyclicpolyolefin (trade name ZEONEX RS820 of ZEON Corporation) having arelative dielectric constant of 2.27 was covered by being extruded, witha draft, at the extrusion temperature of 270° C. from the resindischarge portions defined by the periphery of the central hole 30 a andthe slit holes 30 b. In this way, a generally cross-shaped intermediatemolded component 40 shown in FIG. 8 was obtained.

In this intermediate molded component 40, an annular inner portion 2 ais formed on the outer periphery of the central conductor 1, and fourcoupling portions 2 b are formed extending radially on the outerperiphery of the inner annular portion 2 a.

Next, the intermediate molded component 40 thus obtained was introducedto a circular pipe covering die and covered like a pipe using the sameannular polyolefin thereby to form an insulated covering layer 2 shownin FIG. 9.

The second intermediate molded component 50 formed with the insulatedcovering layer 2 included an inner annular portion 2 a covering theouter periphery of the central conductor 1, four coupling portions 2 bradially extending outward from the outer periphery of the inner annularportion, and an outer annular portion 2 c connecting the outer ends ofthe coupling portions 2 b to each other. The second intermediate moldedcomponent 50 thus had a hollow section with four air gaps 5 at thehollowness of 30% and the outer diameter φ of 0.32 mm.

Next, the second intermediate molded component 50 thus obtained wasetched by an aqueous mixture solution of sulfuric acid, phosphoric acidand chromic acid, sensitized by the hydrochloric acid solution of tinchloride, activated by the hydrochloric acid solution of palladiumchloride, and plated in electroless and electrolytic fashions withcopper thereby to obtain an outer conductor layer 3 having the thicknessof 0.015 mm.

After that, PVC of 0.04 mm thickness was covered as a protectivecovering layer 4. In this way, a small-diameter coaxial cable having anouter diameter φ of 0.43 mm was obtained. In the process, the outerconductor layer 3 formed by plating was sufficiently adhered to theinsulated covering layer 2 and not separated while passing through theguides in the process of forming the protective covering layer 4.

The small-diameter coaxial cable thus obtained had a cross-sectionalstructure as shown in FIG. 1, in which the air gaps occupied 30% of thearea of the insulated covering layer 2, the equivalent dielectricconstant was 1.89 and the characteristic impedance was 50 Ω.

Also, the air gaps 5 were formed at totally inner side of the insulatedcovering layer 2, and therefore not intruded by moisture or the like inthe plating processes, thereby preventing the relative dielectricconstant from rising.

COMPARATIVE EXAMPLE 1

The central conductor (silver-plated copper wire having the outerdiameter φ of 0.1 mm) 1 was heated, so that the surface temperaturebecame 100° C., by a heater using an electric burner and introduced to across head die, and while being taken off at the rate of 30 m/min,cyclic polyolefin (trade name ZEONEX RS820 of ZEON Corporation) having arelative dielectric constant of 2.27 was covered by being extrudedthrough a circular pressure die at the extrusion temperature of 270° C.The covering conductor thus obtained was treated the same way as in thefirst specific example thereby to obtain a small-diameter coaxial cable.

In this small-diameter coaxial cable, the outer diameter of theinsulated covering layer was required to be increased to secure thecharacteristic impedance of 50 Ω, resulting in an increased cable outerdiameter φ of 0.46 mm.

SPECIFIC EXAMPLE 2

The central conductor (silver-plated copper wire having an outerdiameter φ of 0.1 mm) was heated so that the surface temperature became100° C. by a heater using an electric burner, and then introduced to across head die. The central conductor 1, while being inserted throughthe central hole 30 a as in the first specific example, was taken off atthe rate of 30 m/min. At the same time, FEP (trade name NP-100 of DaikinKogyo Co., Ltd.) having a relative dielectric constant of 2.1 wascovered by being extruded at the extrusion temperature of 350° C., witha draft, from the resin discharge portions defined by the periphery ofthe central holes 30 a and the slit holes 30 b. In this way, a generallycross-shaped intermediate molded component 40 shown in FIG. 8 wasobtained.

Next, the intermediate molded component 40 thus obtained was introducedto a circular pipe covering die and covered by extrusion in annularfashion with cyclic polyolefin (trade name ZEONEX RS820 of ZEONCorporation) having a relative dielectric constant of 2.27 at theextrusion temperature of 270° C. thereby to form the outer annularportion 2 c connecting the outer ends of the coupling portions 2 b toeach other. In this way, a second intermediate molded component 50having the cross section shown in FIG. 9 was obtained.

Next, the second intermediate molded component 50 thus obtained wasetched by an aqueous mixture solution of sulfuric acid, phosphoric acidand chromic acid, sensitized by hydrochloric acid solution of tinchloride, activated by the hydrochloric acid solution of palladiumchloride, and plated in electroless and electrolytic fashions withcopper thereby to obtain an outer conductor layer 3 having the thicknessof 0.015 mm. After that, a FEP covering having a thickness of 0.04 mmwas applied as a protective covering layer 4 thereby to obtain asmall-diameter coaxial cable of 0.42 mm in outer diameter. In theprocess, the outer conductor layer 3 formed by plating was sufficientlybonded with the insulated covering layer 2, and not separated whilepassing through the guides in the process of forming the protectivecovering layer 4.

The small-diameter coaxial cable thus obtained had a cross section inthe shape shown in FIG. 1, in which the air gaps 5 represents 30% of theinsulated covering layer 2, the equivalent dielectric constant was 1.82and the characteristic impedance was 50 Ω. Also, as in the firstspecific example, the air gaps 5 were not intruded by water or the likein the plating process nor the relative dielectric constant increased.

The small-diameter coaxial cable thus obtained can be connected with aconnector using solder without melting the insulated covering portion 2.

SPECIFIC EXAMPLE 3

The central conductor (silver-plated copper wire having the outerdiameter φ of 0.1 mm) 1 was heated so that the surface temperaturebecame 100° C. by a heater using an electric burner, introduced to across head die and inserted into a covering die (nozzle) 60 in the shapeshown in FIG. 10.

The die 60 shown in FIG. 10 includes a central hole 60 a for insertingthe central conductor 1 therethrough, and four split holes 60 b formedadjacently to each other on the outer periphery of the central hole 60a. The inner diameter of the central hole 60 a is larger than the outerdiameter of the central conductor 1.

The four split holes 60 b have substantially the same shape and arearranged equidistantly along the peripheral direction around the centerhole 60 a. These generally T-shaped split holes 60 b each include anarcuate portion and a base formed from the center of the arcuateportion.

The edge of the base of each T-shaped split hole 60 b is arranged inproximity to the outer periphery of the central hole 60 a, so that theedges of the arcuate portions arranged adjacently along the peripheraldirection are arranged in proximity to each other. Using the die of thisshape, the central conductor 1 is inserted through the central hole 60a, while at the same time covering by extruding cyclic polyolefin (tradename: ZEONEX RS820 of ZEON Corporation) having a relative dielectricconstant of 2.27 at the extrusion temperature of 270° C. from thecentral hole 60 a and the T-shaped split holes 60 b thereby to form aninsulated covering layer 2 on the outer periphery of the centralconductor 1.

The intermediate molded component 70 formed with the insulated coveringlayer 2, as shown in FIG. 11, includes an inner annular portion 2 acovering the outer periphery of the central conductor 1, four couplingportions 2 b radially extending outward from the outer periphery of theinner annular portion 2 a and an outer annular portion 2 c connectingthe outer ends of the coupling portions 2 b to each other. Theintermediate molded component 70 had a cross section 30% in hollownessand an outer diameter φ of 0.32 mm.

Next, the intermediate molded component 70 thus obtained was etched byan aqueous mixture solution of sulfuric acid, phosphoric acid andchromic acid, sensitized by hydrochloric acid solution of tin chloride,activated by the hydrochloric acid solution of palladium chloride, andplated in electroless and electrolytic fashions with copper thereby toobtain an outer conductor layer 3 having the thickness of 0.015 mm.After that, PVC of 0.04 mm thickness was covered as a protectivecovering layer 4. In this way, a small-diameter coaxial cable having anouter diameter φ of 0.43 mm was obtained.

In the process, the outer conductor layer 3 formed by plating wassufficiently adhered to the insulated covering layer 2 and not separatedwhile passing through the guides in the process of forming theprotective covering layer 3.

The small-diameter coaxial cable thus obtained had a cross-sectionalstructure as shown in FIG. 1. The air gaps occupied 30% in area of theinsulated covering layer 2, the equivalent dielectric constant was 1.89and the characteristic impedance was 50 Ω.

Also, the air gaps 5 were formed totally within the insulated coveringlayer 2, and therefore not intruded by moisture or the like in eachplating process, thereby preventing the relative dielectric constantfrom rising.

SPECIFIC EXAMPLE 4

The central conductor (silver-plated copper wire having an outerdiameter φ of 0.1 mm) 12 was heated so that the surface temperaturebecame 100° C. by a heater using an electric burner, and then introducedto a cross head die. The central conductor 1, while being insertedthrough the central hole 30 a as in the first specific example, wastaken off at the rate of 30 m/min. At the same time, FEP (trade nameNP-100 of Daikin Kogyo Co., Ltd.) having a relative dielectric constantof 2.1 was covered by being extruded at the extrusion temperature of350° C., with a draft, from the resin discharge portion defined by theperiphery of the central hole 30 a and the slit holes 30 b. In this way,a substantially cross-shaped intermediate molded component 40 shown inFIG. 8 was obtained.

The cross section of the intermediate molded component 40 was in theshape of a cross including an annular portion 18 on the outer peripheryof the central conductor 12 an ribs (columnar portions) 20. The ribthickness was 0.06 mm, the rib including the forward thereof has amaximum width was 0.28 mm, and the virtual circular hollow portionformed by connecting the forward ends of the ribs had a hollowness of50%.

Next, 37 silver-plated copper wires constituting strands 24 and havingthe size of 0.03 mm were arranged on the virtual circumferenceconnecting the forward ends of the ribs 20 of the intermediate moldedcomponent 40, which was introduced into a compression die having anouter diameter of 0.34 mm. While rotating the winder, the strands weretwisted to produce a hollow compressed stranded wire. As a result, acoaxial cable 10 comprising an outer conductor layer 16 having an outerdiameter of 0.34 mm was obtained as shown with a roughly illustratedstranded wire in FIG. 2.

Next, the cable 10 thus obtained was introduced to the cross head dieand while being taken off at the take-off speed of 11 mm/min, formedwith a protective cover 26 of FEP resin (trade name: NP-100 of DaikinKogyo Co., Ltd.) having a thickness of 0.04 mm by a covering die. Inthis way, a small-diameter coaxial cable of substantially the samestructure as the small-diameter coaxial cable 10 a shown in FIG. 3 andhaving the final outer diameter of 0.42 mm was obtained.

The characteristic impedance of the small-diameter coaxial cable thusobtained was measured and found to be 50 Ω. Also, the equivalentdielectric constant of the insulated covering layer 14 was 1.55.

COMPARATIVE EXAMPLE 2

As in the specific example 4, a silver-plated copper wire of 0.1 mm wasused as the central conductor 12. To obtain the characteristic impedanceof 50 Ω, the diameter after forming the covering layer was 0.33 mm interms of FEP resin (relative dielectric constant 2.1).

In order to satisfy this specification, the central conductor of 0.1 mmwas introduced to the cross head die and passed through a circularpressure die at the take-off speed of 11 m/min. In this way, FEP resin(trade name: NP-100 of Daikin Kogyo Co., Ltd.) was covered to 0.33 mm atthe extrusion temperature of 350° C.

Next, the shield wire was stranded on this insulated covering conductorhaving an outer diameter of 0.33 mm was stranded at the rate of 2 m/minby a spiral winder. The shield wire was comprised of 38 silver-platedcopper strands of 0.03 mm. As a result, a coaxial cable of 0.39 mm wasobtained comprising the central conductor 12, the insulated coveringlayer and the outer conductor layer.

Next, the cable thus obtained was introduced to the cross head die, andwhile being taken off at the take-off speed of 11 m/min, FEP resin(trade name NP-100 of Daikin Kogyo, Co., Ltd.: relative dielectricconstant 2.1) was covered to the thickness of 0.04 mm, with a draft, bya circular covering die. The final outer diameter was 0.47 mm.

INDUSTRIAL APPLICABILITY

The small-diameter coaxial cable and the method of fabrication thereofaccording to the invention realize a superior, stable high-frequencycharacteristic and electrical characteristics, and therefore caneffectively find applications in reducing the size and thickness ofinformation device terminals such as the notebook-sized personalcomputer.

1. A small-diameter coaxial cable comprising a central conductor, aninsulated covering layer arranged on the outer periphery of the centralconductor and having air gaps continuous along the longitudinaldirection, and an outer conductor layer arranged on the outer peripheryof the insulated covering layer; said insulated covering layer includingan inner annular portion covering the outer periphery of the centralconductor, a plurality of coupling portions extending outward from theinner annular portion and an outer annular portion connecting the outerperipheral edges of the coupling portions to each other, the couplingportions defining the peripheral direction of the air gaps.
 2. Asmall-diameter coaxial cable as described in claim 1, wherein the innerannular portion and the coupling portions combined with the outerannular portion, the inner annular portion combined with the couplingportions and the outer annular portion, or the outer annular portion isformed in two layers of different types of resin.
 3. A small-diametercoaxial cable as described in claim 1 or 2, wherein the outer annularportion is formed of a resin capable of being plated with a metal, andthe outer conductor layer is formed by plating a metal.
 4. Asmall-diameter coaxial cable comprising a central conductor, aninsulated covering layer arranged on the outer periphery of the centralconductor and having air gaps continuous along the longitudinaldirection, and an outer conductor layer arranged on the outer peripheryof the insulated covering layer; said covering layer including anannular portion covering the outer periphery of the central conductor,one or more columnar portions (ribs) extending outward from the annularportion, the outer conductor layer is arranged to be in contact with theouter periphery of the columnar portions, and one or more air gapscontinuous along the longitudinal direction are formed on the inner sideof the outer conductor layer.
 5. A small-diameter coaxial cable asdescribed in claim 4, wherein the outer conductor layer is formed of ahollow compressed stranded wire.
 6. A small-diameter coaxial cable asdescribed in claim 4, wherein the outer conductor layer is formed bywinding the outer periphery of the columnar portions with a metal tapeor a metal foil of superior electrical conductivity such as copper or ametal laminate film including the metal tape or the metal foil laminatedwith a plastic film.
 7. A small-diameter coaxial cable as described inclaim 4, wherein the outer conductor layer is formed of a metal pipesuperior in electrical conductivity such as copper, and a semi-finishedproduct (insulated core) formed with a covering layer having thecolumnar portions is inserted into the metal pipe while drawing andextending the metal pipe through a die.
 8. A small-diameter coaxialcable as described in any one of claims 1, 2 or 4, wherein a pluralityof the coupling portions and a plurality of the columnar portions areextended radially at equal angular intervals in the cross section whileat the same time being extended along the longitudinal axial directionof the small-diameter coaxial cable with the same intervals.
 9. Asmall-diameter coaxial cable as described in any one of claims 1, 2 or4, wherein the coupling portions and the columnar portions are formedspirally along the longitudinal direction.
 10. A small-diameter coaxialcable as described in any one of claims 1, 2 or 4, wherein the annularportion, the coupling portions and the columnar portions are formed byextruding fluoro resin such as FEP, PFA or PTFE or synthetic resin suchas APO (amorphous polyolefin) or PEN (polyethylene naphthalate).
 11. Asmall-diameter coaxial cable as described in any one of claims 1 or 4,wherein the insulated covering layer occupies not less than 10% of thearea of the air gaps in the cross section.
 12. A small-diameter coaxialcable as described in any one of claims 1 or 4, wherein a protectivecovering layer is formed on the outer periphery of the outer conductorlayer.
 13. A method of fabricating a small-diameter coaxial cable,comprising: a covering die including a central hole for insertion of thecentral conductor therethrough and a resin discharge portion having acircular annular portion formed on the outer periphery of the centralhole and a plurality of radial slits extending radially outward from theouter periphery of the circular annular portion is used in such a mannerthat the central conductor is inserted through the central hole while atthe same time molding by extruding the melted thermoplastic resin, witha draft, from the resin discharge portion thereby to obtain anintermediate molded component including an inner annular portioncovering the outer periphery of the central conductor and a plurality ofcoupling portions extending outward from the inner annular portion andsimilar in shape to the die, after which the intermediate moldedcomponent is introduced to the head of a melt extruder, and the outerannular portion is covered by extrusion on the coupling portions by anannular covering die thereby to form the insulated covering layer havingthe air gaps, after which the outer conductor layer and the protectivecovering layer are sequentially formed on the outer periphery of theinsulated covering layer.
 14. A method of fabricating a small-diametercoaxial cable, that comprising: a central conductor covered by anextrusion with the thermoplastic resin melted in annular fashion, with adraft, by an annular covering die thereby to obtain an intermediatemolded component having an inner annular portion covering the outerperiphery of the central conductor, after which using a die including acentral hole, an annular portion and a resin discharge portion having aplurality of radial holes extending radially from the inner periphery ofthe annular portion, the intermediate molded component is insertedthrough the central hole while extruding the melted thermoplastic resinfrom the resin discharge portion with a draft thereby to form an outerannular portion and a plurality of coupling portions extending to thecenter, thereby forming the insulated covering layer having the airgaps, after which the outer conductor layer and the protective coveringlayer are sequentially formed and covered on the outer periphery of theinsulated covering layer.
 15. A method of fabricating a small-diametercoaxial cable as described in claim 14, wherein in place of the processof obtaining the intermediate molded component, a dispersion in whichthe thermoplastic resin particles is dispersed in a dispersion medium(liquid) is coated or impregnated around the central conductor, afterwhich the dispersion medium is evaporated thereby to form an annularcovering on the central conductor or an annular covering is formed bypowder coating thereby to form the inner annular portion and obtain anintermediate molded component having the inner annular portion coveringthe outer periphery of the central conductor.
 16. A method offabricating a small-diameter coaxial cable comprising a centralconductor, an insulated covering layer arranged on the outer peripheryof the central conductor and having air gaps continuous along thelongitudinal direction, an outer conductor layer arranged on the outerperiphery of the insulated covering layer and a protective coveringlayer arranged on the outer periphery of the outer conductor layer,comprising: using a die having a central hole for inserting the centralconductor therethrough and a plurality of T-shaped split holes arrangedadjacently to each other on the outer periphery of the central hole, thecentral conductor is inserted through the central hole while at the sametime extruding the melted resin from the central hole and the T-shapedsplit holes thereby to form the insulated covering layer having the airgaps continuous along the longitudinal direction on the outer peripheryof the central conductor, after which the outer conductor layer and theprotective covering layer are sequentially formed and covered on theouter periphery of the insulated covering layer.
 17. A method, offabricating a small-diameter coaxial cable as described in any one ofclaims 13 to 16, wherein the outer conductor layer is formed by platinga metal.
 18. A method of fabricating a small-diameter coaxial cable,comprising a covering die including a central hole for inserting acentral conductor therethrough and a resin discharge portion having anannular portion and a plurality of radial slits extending radiallyoutward from the outer periphery of the annular portion is used in sucha manner that the central conductor is inserted through the central holewhile at the same time molding by extrusion, with a draft, the meltedthermoplastic resin from the resin discharge portion thereby to obtainan intermediate molded component (insulated core) similar in shape tothe die and having an inner annular portion covering the outer peripheryof the central conductor and a plurality of coupling portions extendingoutward from the inner annular portion, which intermediate moldedcomponent is supplied continuously so that an outer conductor layer isformed by covering a hollow compressed stranded wire or winding a metalfoil, a laminate film or the like or covering by extending a copper pipeon the outer periphery of the columnar portions, after which an outercovering layer is formed on the outer periphery of the outer conductorlayer.